Amino resins, especially acetalized or etherified melamine/formaldehyde resins, and their use as crosslinking agents in thermally curable materials, especially coating materials, have been known for some time (cf., for example, Johan Bieleman, “Lackadditive”, “7.2.2 Melamine resin-crosslinking systems”, Wiley-VCH, Weinheim, N.Y., 1998, pages 242 to 250). The coating materials in question are known to give coatings having a high surface hardness.
The stability of these coating materials on storage, however, is limited, owing to the presence therein of acid catalysts.
Moreover, the amino resins have a propensity toward self-condensation, thereby raising the brittleness of the coatings and reducing their elasticity.
The amino resins, particularly the etherified melamine/formaldehyde resins, are also used as co-crosslinking agents in coating materials which are prepared from what are known as multicomponent systems, especially two-component systems. As is known, the multicomponent systems include at least one binder component, which comprises preferably hydroxyl-containing constituents, and at least one crosslinking component, which comprises preferably polyisocyanates. The two components are stored separately from one another and mixed with one another only a short time before being used to prepare the actual coating material. The coatings produced from these coating materials are known to have advantageous combinations of properties, such as good etch resistance and good wet scratch resistance.
However, in the course of the thermal curing of these coating materials, alcohol bound in acetal form can be eliminated from the amino resins, thereby unpredictably altering the original equivalent ratio of hydroxyl groups to isocyanate groups. Then, by reaction with the polyisocyanates, the liberated alcohol automatically forms network endgroups in the resulting thermoset material, which can disadvantageously reduce the network density and hence the scratch resistance of the coatings in question.
The modification of amino resins with compounds containing a carbamate group, such as methyl carbamate, for example, is known from the American patent application US 2005/0182189 A1. In this case the resulting amino resins contain groups of the formula >N—CH2—NH—C(O)—OCH3. However, on account of an absence of isocyanate-reactive functional groups, and of their high melting point and high crosslinking temperatures, these amino resins are unsuited to use as constituents of multicomponent systems.